| Chromium-containing slag is a kind of the poisonous slag generated from chromate industry.The accumulated amount of chromium-containing slag in China was more than 4 million tons and 600 thousand tons are being discharged annually.Water soluble Cr(Ⅵ) in chromium-containing slag is a carcinogenic substance,which caused a seriously environmental threat and has become a urgent problem to be solved.Since two serious accident occurred by chromium-containing slag in 2003,much more attentions were paid on the detoxification of chromium-containing slag.Generally,the detoxification of chromium-containing slag includes dry-detoxification and wet-detoxification.The above two methods was not widely applied because of high cost or incomplete detoxification that results in the secondary pollution.Therefore,the pollution problem of the chromium-containing slag is still not solved.In this dissertation,the characteristics of chromium-containing s lag and its leaching were investigated in order to seek an effective and cheap method for detoxification of chromium-containing slag. Chromium-containing slag is a kind of hazardous solid waste with complicated phase composition.Chromium-containing slag contains both water-soluble Cr(Ⅵ) that is susceptible to leached out and aci d-soluble Cr(Ⅵ) that is difficult to be leached out.It was found th at temperature,particle diameter,stirring speed obviously affected Cr(Ⅵ) leaching ratio in shaking experiment.In water leaching expe riment,the leaching velocity was inversely proportional to the ratio of solid to liquid.In acid leaching experiment,the optimum condit ion for Cr(Ⅵ) leaching by HCl were pH 3.0,5:1 of liquid to solid ratio,180mL/min of stirring speed,40℃of temperature.The kinet ics of Cr(Ⅵ) leaching by HCl can be described with the followin g model:v=11.58t-0.54.And the kinetics model of Cr(Ⅵ) leaching by NaCl were:v=3.38×10-7 cNacl0.26 with the apparent activation energy of 34.24kJ/mol.The Cr(Ⅵ) leaching velocity was obviously influen ced by temperature and the relationship between k and temperature can be described with following equation:k(T)=exp(13.71—4117.9/ T).The results can provide theoretic basis to select technique and i ts parameters for the detoxificationA bacterial strain that can reduce high concentrations of Cr(Ⅵ) in alkaline medium was isolated from chromium-containing slag site.It was identified as Achromobacter sp.and nominated as CH-1.Strain CH-1 grew in alkaline(8＜pH＜11) and aerobic condition,middle temperature (25-35℃) and organic carbon and organic nitrogen as nutrient source. Strain CH-1 was resistant to 20 g/L salt and 4 g/L Cr(Ⅵ).Its ability for Cr(Ⅵ) reduction reached up tol.5g/L.Moreover,cations showed significantly inhibitory effect on Cr(Ⅵ) reduction than anions.Blue Cr(OH)3 precipitate was observed,during Cr(Ⅵ) reduction by CH-1.The reaction of Cr(Ⅵ) reduction by CH-1 was zero reaction.The detoxification of Cr(Ⅵ) containing slag with Achromobacter sp. CH-1 was carried out.The results revealed that Cr(Ⅵ) removal reached up to 88.69%in shaking experiment,implying that detoxification of chromium-containing slag with Achromobacter sp.CH-1 was practically feasible.Moreover,a novel technique of "granulation of chromium-containing slag-microbial detoxification-chromium recovery" was put forward in this study.In column leaching experiment,Cr(Ⅴ) reaching ratio was 92.088%under the optimum condition.Cr(Ⅵ) concentration in leachate of the detoxified slag met the waste water discharge standard(GB 8978-1996).The detoxified slag met the identification standard for hazardous waste- identification for extraction toxicity(GB 5085.3-2007).Macroscopic mechanism of microbial-detoxification of chromium containing slag was also studied in this study.The results showed that Cr(Ⅵ) was leached from chroraium-containing slag followed by the direct reduction by bacterial stains.Consequently,the phase composition of slag was decomposed by bacterial stains and this erosion action slowly extended into interspaces between slag particles,which further promoted the proceedings of erosion.The precipitation of Cr(OH)3 was obtained during Cr(Ⅵ) reduction processes.Ca12Al14O33 and Ca4Al2SO10·16H2O in the detoxified slag disappeared.The content of periclase substantially decreased.Consequently,the enwrapped acid soluble Cr(Ⅵ) was released from the above chromium-containing substances.Thus,the key point of microbial-detoxification of chromium-containing slag was to enhance the leaching ratio of acid soluble Cr(Ⅵ).The pilot experiment for the detoxification of chromium-containing slag was carried out using the technique of "granulation of chromium-containing slag-microbial heap leaching".The project run 10t/batch and 20t/batch,respectively.Based on the optimization of technique parameters,chromium-containing slag was completely detoxified after running 7-10 days.The detoxified slag met the identification standard for hazardous waste- identification for extraction toxicity(GB 5085.3-2007) Meanwhile,about 90%of Cr(Ⅵ)was recovered.It was approximately estimated that the expenses of microbial detoxification for chromium-containing slag was 200 yuan per ton slag. The results verified that it is practically feasible on full scale.The outcome provides a new approach for the detoxification of chromium-containing slag.
|
PDF Full Text Request